In the processing of biological fluids such as blood where a patient is connected to the blood processing apparatus as part of a closed system, one of the major objectives is to maintain a constant flow rate of blood to the processing apparatus under conditions which include pressure variation from positive to negative at the input of a peristaltic pump. The pressure variations can arise from a number of sources including blood pressure drops in the patient and may vary quite widely in the course of a blood processing run.
The variations in flow rate due to pressure variations can be minimized by either using stiffer tubing or by restricting the allowable input pressure. The former approach, however, increases the torque requirements on the pump which, in turn, requires a larger motor while the latter approach may not be possible because of system requirements.
U.S. Pat. No. 3,882,861 filed Sept. 24, 1973 and issued May 13, 1975 shows an auxiliary control for a blood pump which acts on an existing blood pump controller to continuously vary the pump rate in direct correspondence with changes in flow of a patient's blood reflected as variations in negative pressure in a blood line. In this patent, the the negative pressure is sensed upstream from the pump such that when blood flow decreases (greater negative pressure is sensed), the speed of the pump is decreased and, when blood flow increases (lesser negative pressure is sensed), the speed of the pump is increased. Thus, pump starvation caused by vacuum-induced collapse of blood lines and blood vessels is avoided because the pump will operate only at the maximum rate accommodated by the available blood supply. The present invention differs from the patent in that it seeks to maintain a constant flow rate to the input of a pump the flow rate of which changes in accordance with the pump characteristic when the input pressure to the pump enters a negative pressure regime. The patent doesn't appear to be constrained in any way by the pump characteristic and reduces speed when blood flow decreases and increases speed when blood flow increases while, in the present invention, pressure is sensed at the input of the pump and the pump speed is increased in the face of decreasing blood flow. The patent seeks to maintain a constant negative pressure whereas the present invention seeks to maintain a constant flow rate regardless of the variation in negative pressure.
U.S. Pat. No. 4,055,175 filed May 7, 1976 and issued Oct. 25, 1977 shows a sensor, computer and pump system whereby insulin is infused by a pump into a patient under control of a computer which is programmed to provide desired output signals based on sensed blood glucose concentration. In the patent, neither pressure nor flow rate in the pump is continuously controlled. Only the glucose infusion rate is controlled with no consideration being given to the pump characteristic, if any.
U.S. Pat. No. 4,080,966 filed Aug. 12, 1976 and issued March 28, 1978 shows supplying a hypertensive control agent in response to increased blood pressure by a controller. The output of the latter controls a pump. There is no teaching of sensing pump input pressure nor is any means shown for maintaining a constant flow rate.
U.S. Pat. No. 3,946,731 filed July 31, 1974 and issued March 30, 1976 shows a blood treatment system where pressure is sensed downstream of the pump and a signal applied to a controller which controls the rate of pumping in the event the sensed pressure moves towards one or the other of preselected limits. In this patent, the intent appears to be to maintain a constant pressure at the pump input whereas, in the present invention, the intent is to maintain the flow rate constant regardless of pressure variation.
It is, therefore, an object of the present invention to provide a microprocessor controlled system in which flow rate of blood or other biological fluid through a peristaltic pump is maintained constant regardless of variations in negative pressure at the pump input.
Another object is to provide a system for pumping blood in which the pressure vs flow rate characteristic of a pump in the negative pressure regime is compensated for by increasing pump speed in accordance with the relationship ##EQU1## Still another object is to provide a system for pumping blood which permits control of flow rate in real time by means of a microprocessor which is responsive to sensed input pump pressure, a pressure determined from the pump characteristic and a desired flow rate.